Conventionally, as an image forming apparatus, such as a printer, a copier, an image transmitting/receiving apparatus, and a multi-function apparatus that is provided with functions of such apparatuses, there is an apparatus that have a print head configured provided with a liquid discharging head for discharging ink droplets of recording liquid (hereinafter, referred to as ink) in a liquid form and performs an image forming (i.e. recording, printing, imaging, or typing) process by attaching ink to a medium (hereinafter, the medium is also referred to as a sheet, the material thereof is not limited thereto, and a recorded medium, a transfer sheet, a recording sheet, or the like means the same) while conveying the medium.
In the following description, the image forming apparatus means an apparatus that discharges liquid onto a medium such as paper, thread, textiles, cloth, leather, metal, plastic, glass, wood, or ceramics. In addition, “image forming” means not only providing an image having a meaning such as a text or a diagram to a medium but also providing a meaningless image such as a pattern to a medium (broadly meaning: the image forming apparatus means the same as a liquid discharging apparatus in the meaning of an apparatus that discharges liquid droplets).
As a liquid discharging head, a piezoelectric type ink-jet print head is known, which uses a so-called piezoelectric actuator which has a piezoelectric element as a pressure generator for generating pressure to press ink inside a liquid chamber that is connected with nozzles used for discharging liquid droplets and changes the volume/pressure inside the liquid chamber so as to discharge the liquid droplets by transforming an elastically-transformable member (diaphragm) forming one side of the liquid chamber.
In a multi-nozzle ON-Demand type ink-jet print head that is provided with a number of nozzles, it is important to decrease non-uniformity of discharging speeds and masses of ink droplets in each of the nozzles for performing a recording process to obtain a high-quality image at high to speed with high reliability.
The ON-Demand type ink-jet print head uses a push-type piezoelectric element in which the wall of an ink pressing chamber having a nozzle opening is configured by a diaphragm, the diaphragm is pressed by vertical vibration of a rod-shaped piezoelectric element, and ink droplets are discharged by increasing the pressure of the ink inside the ink pressing chamber. In the ON-Demand type ink-jet print head, a plan for increasing precision of the piezoelectric element and components constituting the ink pressing chamber, and improving the assembly precision for bonding the components has been performed.
However, there is a case where such a plan is accompanied by a problem such as an increase of costs of the components or an increase of the assembly time.
Accordingly, as a method for appropriately adjusting the polarization state of the piezoelectric element, a so-called polarization correcting method for correcting the non-uniformity of the droplet discharging speeds or droplet masses among the nozzles to be decreased has been known. As a method of appropriately adjusting the polarization state of the piezoelectric element, it is known that a method in which a polarization voltage applied to a piezoelectric element of a nozzle discharging high-speed droplets is adjusted and set to be lower than that of a nozzle discharging low-speed droplets is simple and convenient. An example of such method is disclosed in JP-A-2001-277525.
By using this method, an ink-jet print head is obtained by not requiring an additional part, circuit, or the like, of which non-uniformity of discharging speeds or masses of the ink droplets is improved at high precision in an easy manner by only adding adjustment costs in the manufacturing process of the head.
However, in a polarization correcting method, as described in the document JP-A-2001-277525, in which the polarization states of the piezoelectric elements are corrected by adjusting the polarization voltages in accordance with the ink droplet discharging speeds of the entire nozzles, for a print head of which non-uniformity of ink droplet discharging speeds is high, the amount of adjustment of the polarization voltages needs to be large, and accordingly there is a case where the polarization voltage becomes too low.
When the polarization voltage is too low, the piezoelectric elements are polarized by driving voltages applied to the piezoelectric elements for discharging ink droplets, the result of adjustment of the degree of polarization becomes unsuccessful in accompaniment with elapse of a driving time for discharging ink droplets, and thereby the irregularity of discharging speeds among the nozzles becomes high. Accordingly, it has been determined that there is a problem that a correction process having usable endurance cannot be performed for a print head having high non-uniformity of ink droplet discharging speeds. In addition, this problem occurs in the same manner by performing a correction process for the non-uniformity of the ink droplet masses.